Arc tubes or arc vessels for high-pressure discharge lamps may use ceramic discharge vessels to enclose the arc under high pressure conditions. Such vessels are used for sodium high-pressure discharge lamps and, recently, the advantages of such vessels have led to their use in metal halide discharge lamps. These lamps, customarily, have a discharge vessel made of quartz glass. To improve the color rendition of the light output, higher operating temperatures than those which can be tolerated by quartz glass are necessary and, therefore, ceramic enclosures for the arc tubes or discharge vessels are desirable. The power ratings involved are between about 50 to 250 W. Customarily, the discharge vessels are elongated and have two ends which are of tubular shape. Current supply leads pass through the ends, for example through cylindrical, ceramic plugs closing the ends. The current supply leads, to which electrodes can be attached, or which have electrodes at their ends, then pass through the end plugs.
Tubular, as well as pin or rod-like lead-throughs of niobium have been used in sodium high-pressure discharge lamps, see for example British Patent 1,465,212, Rigden, and U.S. Pat. No. 4,376,905, Kerekes. The niobium pin or tube is melt-sealed in a ceramic end plug by a glass solder or by a melt ceramic. It has also been proposed (see U.S. Pat. No. 4,545,799, Rhodes et al, to which European Patent 0 136 505 corresponds) to utilize sintering technology for the niobium tube, to provide a direct lead-through connection which is free of glass melt or glass solder. Sodium high-pressure discharge lamps have a peculiarity in that the fill includes a sodium amalgam which frequently is retained in a reservoir in the interior of the niobium tube used as a lead-through for electrical energy into the interior of the vessel.
A particularly simple solution to fill and evacuate the discharge vessel utilizes a small opening formed in one of the two niobium tubes fitted in the respective ends of the discharge vessel. The small opening is located in the vicinity of an electrode shaft and open to the interior of the discharge vessel. Evacuation of the interior of the vessel and fill of the amalgam and introduction of an inert gas is possible through such a tube, see U.S. Pat. No. 4,342,938, Strok, to which British Patent 2,072,939, Strok, corresponds. After filling the lamp, the end of the niobium tube which extends beyond the discharge vessel is closed by a pinch or press operation, and then welded, to be gas-tight. This leaves the opening in the interior of the vessel, in the vicinity of the electrode shaft, still free. During operation, a connection is available between the interior of the discharge vessel and the interior of the lead-through tube, forming a cold spot.
U.S. Pat. No. 4,011,480, Jacobs et al, to which German Patent 25 48 732, corresponds, describes another technology to close off the open ends of a sodium high-pressure discharge lamp. Tubular current lead-throughs of tungsten, molybdenum or rhenium are used, melt-sealed in the plug closing off the vessel openings. A ceramic, cylindrical pin located in the interior of the tube is melt-sealed, gas-tightly in the tubular lead-through. The external end of the tube is not pinched or closed after termination of the filling process. Since the metals used, tungsten, molybdenum or rhenium, are brittle, they can be worked on only with great difficulty, in contrast to niobium. Thus, termination technologies which are known and have been used with niobium tubes cannot be readily transferred for use with the brittle materials. Rather, the ceramic central pin is formed with an axial bore which, upon evacuation and filling, cooperates with an opening in the tube in the vicinity of the electrode shaft. After the filling, the axial bore of the central pin is closed by a melt ceramic. Thus, the brittle metal need not be worked on.
This technology is difficult to carry out, time-consuming and expensive in production.
U.S. application Ser. No. 07/912,526, filed Jul. 13, 1992, Bunk et al, now U.S. Pat. No. 5,404,078, Apr. 4, 1995, assigned to the assignee of the present application (the disclosure of which is published in European Application 92 114 227.9) discloses a high-pressure discharge lamp with a ceramic discharge vessel having a tubular current supply of molybdenum or the like, which is directly sintered in a ceramic end plug. International Application PCT/DE 92/00372, in which the United States is designated, U.S. application Ser. No. 08/211,608, filed Apr. 7, 1994, Heider et al, now U.S. Pat. No. 5,484,315, Jan. 16, 1996, assigned to the assignee of the present application, discloses a method of filling a metal halide discharge lamp having a ceramic discharge vessel in which an opening is formed in either the lead-through or in the discharge vessel or in a closing plug to form a bore for a fill. As described, two ends of the discharge vessels are fitted with electrode systems and sealed, with a filling bore near the pump end of the tube remaining open, the bore being closed after the tube has been filled. The additional opening in the discharge vessel or in the end plug must first be formed and then closed. If the opening is made in a tubular lead-through, the electrode secured to the tubular lead-through decreases the clear diameter of the tubular lead-through available for introducing the fill, and may seriously interfere with the filling process. Closing off the additional filling opening or bore is time-consuming and requires sophisticated techniques.
Direct sintering of lead-throughs, when used for sodium high-pressure discharge lamps, has a problem in that, during sintering, temperatures of more than 1800.degree. C. arise. Usually, the electrodes of such lamps include emitter material which may vaporize at the high sintering temperature.
For automatic manufacture in series or mass production, it is necessary that electrode systems are available at the time the end plugs are sintered. If the individual process steps are carried out at different locations, transport of the electrode systems to the vessels is necessary, and the possibility of damage to the electrode systems during transport is considerable. Flexibility with respect to the use of specific electrodes and electrode systems in particular vessels also is limited.